replay: don't process events at virtual clock checkpoint

As QEMU becomes more multi-threaded and non-synchronized, checkpoints
move from thread to thread. And the event queue that processed at checkpoints
should belong to the same thread in both record and replay executions.
This patch disables asynchronous event processing at virtual clock
checkpoint, because it may be invoked in different threads at record and
replay. This patch is temporary fix until the checkpoints are completely
refactored.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Message-Id: <20181018063345.7433.11678.stgit@pasha-VirtualBox>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

replay/replay-events.c

@@ -190,6 +190,7 @@ void replay_save_events(int checkpoint)
 {
     g_assert(replay_mutex_locked());
     g_assert(checkpoint != CHECKPOINT_CLOCK_WARP_START);
+    g_assert(checkpoint != CHECKPOINT_CLOCK_VIRTUAL);
     while (!QTAILQ_EMPTY(&events_list)) {
         Event *event = QTAILQ_FIRST(&events_list);
         replay_save_event(event, checkpoint);

replay/replay.c

@@ -214,7 +214,14 @@ bool replay_checkpoint(ReplayCheckpoint checkpoint)
         /* This checkpoint belongs to several threads.
            Processing events from different threads is
            non-deterministic */
-        if (checkpoint != CHECKPOINT_CLOCK_WARP_START) {
+        if (checkpoint != CHECKPOINT_CLOCK_WARP_START
+            /* FIXME: this is temporary fix, other checkpoints
+                      may also be invoked from the different threads someday.
+                      Asynchronous event processing should be refactored
+                      to create additional replay event kind which is
+                      nailed to the one of the threads and which processes
+                      the event queue. */
+            && checkpoint != CHECKPOINT_CLOCK_VIRTUAL) {
             replay_save_events(checkpoint);
         }
         res = true;